Hydraulic power transmission systems are generally useful for providing useful work on the actuation of a load. The actuation of the load may be solely provided by high pressure hydraulic fluid or may be provided by mechanical apparatus with the hydraulic system providing servo assist to the load actuation. In either type of system, a pump draws hydraulic fluid from a reservoir and pumps such fluid through a high pressure supply line to the load wherein the high pressure fluid may perform work. The fluid is thereafter returned to the reservoir through a low pressure return line.
A motor vehicle power steering system is one particular example of the hydraulic servo assist system in that steering actuation of the tires is provided primarily through a mechanical connection between the steering wheel and the steering tires. In one particular example, the steering wheel is connected at one end of a steering column and a pinion is coaxially carried by the other end of the steering column. The pinion drives a steering rack wherein rotation of the pinion, by turning the steering wheel, is translated into a linear displacement of the steering rack. The steering rack is in turn connected to the tie rod which interconnects the steerable spindle/hub assembly of the vehicle suspension system. The steering tires are mounted to the hub. The combination of the rack and pinion are referred to as a steering gear. Another type of steering gear is the recirculating ball.
In the hydraulic servo assist, the hydraulic pressure acts on the steering gear (in the rack and pinion gear with the resultant force being in the direction of movement of the rack) to reduce steering effort required at the steering wheel to move the steering wheels from the straight ahead position. A typical suspension biases the spindle/hub so that the steering tires return to the straight ahead position in absence of any further steering input. The amount of servo assist is selected to maintain "road feel" of the steering tires at the steering wheel. Usually, excessive assist diminishes road feel and insufficient assist causes unacceptably high steering effort.
The hydraulic pressure in the high pressure supply line required to provide the desired amount of servo assist, or generally in any type of hydraulic power transmission system the desired hydraulic force, is defined as the static hydraulic pressure of the system. By maintaining the static pressure of high pressure supply constant, linearity of the work which the hydraulic fluid performs on the load is ensured. In the particular example of the motor vehicle hydraulic servo assist system, the steering effort required at the steering wheel is strongly dependent on vehicle road speed with the required assist decreasing with increasing road speed. Accordingly, the static pressure in such systems is desirably made an inverse function of road speed, and the prior art has addressed this aspect of static pressure control.
The hydraulic pressure in the high pressure supply line of the power transmission system will also exhibit transients in the static pressure during operation of the system. These transients are a result of conditions at and acting on the load, in the pump, at discontinuities to fluid flow such as at valves and junctions, and any other condition known in fluid dynamics to cause unwanted fluid oscillations and wave energy and oscillations. These transients define the dynamic pressure of the hydraulic fluid in the high pressure supply line.
In the motor vehicle servo assist system, the dynamic pressure is a result of several unrelated and unpredictable occurrences. For example, during straight ahead driving, a hydraulic valve at the servo assist is closed so that strong "on center" feel is achieved. At the moment there is a steering input, this valve is opened and the fluid directed to perform work acting on the steering rack in the direction of the movement. The opening, and subsequent closing of this valve upon returning to straight ahead, develops pressure transients in the high pressure supply line which can be felt at the steering wheel as kickback, thereby degrading true road feel.
Furthermore the hydraulic fluid pump is typically belt driven from the engine which does not operate at constant speed. Accordingly, the pumping pulses applied to the high pressure line vary with engine, and not road, speed such that a transmission shift during a turn at constant road speed will cause a change in steering feel due to the change of engine speed. In addition, road irregularities encountered at either steering wheel will transmit shock and vibrations to the steering rack which in turn develop further dynamic pressure in the high pressure line. Road irregularities and shock are felt as at the steering wheel if the dynamic pressure is not properly damped. Also, chassis dynamics of the vehicle will cause slight variations in alignmnet and placement of the various steering system components with respect to each other. These variations will also introduce unwanted dynamic pressure.
A prior art solution to provide for the damping of the dynamic pressure, regardless of its source, is to install a reservoir in the high pressure line to act as a resonant cavity to tune the high pressure fluid to low frequencies so that the static pressure is undisturbed and the dynamic pressure is damped. However, a disadvantage and limitation of such a cavity is that the overall bandwidth of the hydraulic system is accordingly reduced. Therefore, response of the hydraulic system is degraded. In the motor vehicle servo assisted steering, degraded hydraulic response will cause steering assist to be applied too slowly during rapid steering maneuvering, such as during a swerve and recovery.